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Debt (Narrative) (Details) - USD ($)
6 Months Ended
Aug. 08, 2019
Jun. 30, 2020
Aug. 08, 2026
Aug. 08, 2025
Aug. 08, 2024
Dec. 31, 2019
Dec. 15, 2015
Jul. 30, 2015
Debt Instrument [Line Items]                
Document Period End Date   Jun. 30, 2020            
Debt Instrument, Interest Rate, Stated Percentage 4.00%              
Proceeds from Issuance of Senior Long-term Debt $ 73,300,000              
Debt Instrument, Fee Amount 1,500,000              
Payments of Debt Issuance Costs 200,000              
Costs associated with amendment   $ 1,125,000       $ 1,358,000    
Borrowings on lines of credit   143,875,000       164,642,000    
Line of Credit Facility, Remaining Borrowing Capacity   26,100,000            
Senior Notes $ 75,000,000.0 73,527,000       73,406,000    
Letters of Credit Outstanding, Amount   9,000,000.0            
Unsecured Debt [Member]                
Debt Instrument [Line Items]                
Line of Credit Facility, Maximum Borrowing Capacity   275,000,000.0            
Borrowings on lines of credit   $ 215,000,000.0         $ 40,000,000.0  
Unsecured Debt [Member] | Minimum [Member]                
Debt Instrument [Line Items]                
Line of Credit Facility, Interest Rate at Period End   2.67%            
Unsecured Debt [Member] | Maximum [Member]                
Debt Instrument [Line Items]                
Line of Credit Facility, Interest Rate at Period End   2.68%            
Revolving Credit Facility [Member]                
Debt Instrument [Line Items]                
Line of Credit Facility, Maximum Borrowing Capacity   $ 35,000,000.0           $ 50,000,000.0
Borrowings on lines of credit   $ 15,000,000       $ 38,000,000    
Line of Credit Facility, Interest Rate at Period End   4.00%            
Letters of Credit Outstanding, Amount   $ 8,900,000            
Forecast [Member]                
Debt Instrument [Line Items]                
Long-term Debt, Maturities, Repayments of Principal in Rolling after Year Five     $ 50,000,000.0 $ 12,500,000 $ 12,500,000